|Portis Jr, Archie|
Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/2006
Publication Date: 9/22/2006
Citation: Salvucci, M.E., Deridder, B.P., Portis Jr, A.R. 2006. Effect of activase level and isoform on the thermotolerance of photosynthesis in arabidopsis. Journal of Experimental Botany. 57:3793-3799. Interpretive Summary: In the process of photosynthesis, plants convert light into chemical energy. The energy produced by photosynthesis is then used to synthesize sugars and other foodstuffs. Heat stress inhibits photosynthesis, reducing the overall yield of the plant. Previous research from this research group identified an enzyme called Rubisco activase as the component of photosynthesis that is most sensitive to inhibition by heat. Rubisco activase is a regulatory enzyme that controls the activity of the major carbon dioxide-fixing enzyme in plants. In this manuscript, we present new data that provides additional details to the biochemical mechanism for inhibition of photosynthesis by moderate heat stress. Specifically, we use transgenic plants to show that the amount of activase, rather than the specific form of activase, is an important determinant of plant thermotolerance. The results provide new insights into the process by showing that conditions in the chloroplast are less important for thermal stability than the inherent properties of activase. This information eventually can be used to make changes that improve the activity and stability of Rubisco activase in order to improve the efficiency of photosynthesis under heat stress.
Technical Abstract: Rubisco activation decreases under moderate heat stress. This decrease is caused by an impairment of activase function, which is exacerbated by faster rates of Rubisco deactivation at elevated temperatures. To determine if stromal oxidation causes inhibition of activase, transgenic Arabidopsis plants expressing similar, but suboptimal amounts of either the redox-regulated 46 kD alpha- or non-redox regulated 43 kD beta-isoform of activase were examined. Photosynthesis, as measured by gas-exchange and chlorophyll fluorescence, and Rubisco activation were inhibited to a much greater extent by moderately high temperatures in the two transgenic lines expressing suboptimal levels of the individual isoforms of activase compared with wild type plants or transgenic plants expressing near wild-type levels of the beta-isoform. Net photosynthesis and Rubisco activation in transgenic plants expressing suboptimal amounts of the beta-isoform of activase from the Antarctic hairgrass were even more sensitive to inhibition by moderate heat stress than in the transgenic plants containing Arabidopsis activase. The results demonstrate that photosynthesis is equally sensitive to inhibition by moderately high temperature in plants expressing the two different isoforms of activase. Thus, impairment of activase function under heat stress is not caused by oxidation of the redox-sensitive sulfhydryls of the alpha-isoform of activase. Instead, the results are consistent with thermal denaturation of activase under moderate heat stress, the effects of which on Rubisco activation would be enhanced when activase levels are suboptimal for photosynthesis.